I had 7 or 8 left. It's getting easier with every time you try it and in my opinion the final boss is one of the easiest in the game in a way. I just loved the Meta Ridley and Metroid Prime fights best in the games - those were old school boss battles where you were able to take a beating too, unlike many of the other fights where the bosses dealt out massive damage with every attack but couldn't take any, giving you little time to work out a strategy.